It's no news flash that consuming caffeine close to bedtime can interfere with sleep. But scientists at the University of Colorado Boulder report in the journal Science Translational Medicine that caffeine has another physiological impact on sleep by delaying the body's natural surge in the production of the sleep hormone melatonin, which in turn pushes back the body's circadian clock.

"To our surprise, no one had really tested this question," one of the researchers tells NPR. "What we're seeing here now is another way that caffeine impacts our physiology that we didn't know about before in humans." The problem in this case is that the circadian clock is "present in cells throughout our entire body," says the researcher.

"It's in your fat cells; it's in your muscle cells." Messing with it appears to play a role in a wide range of health problems, from obesity to cancer.

Many studies have recently suggested that bright light at night, especially the blue light emanating from our screens, impedes melatonin production. Caffeine's effect is about half what scientists found when exposing volunteers to bright light alone; volunteers who drank a double espresso three hours before bedtime experienced a 40-minute delay of the melatonin surge.

Now the researchers plan to study whether caffeine consumption five or six hours before bedtime produces a similar effect. (Still want to watch TV at night? Orange glasses should do the trick.)